1. Field of the Invention
The present invention relates to a laser beam optical scanning device and an image forming apparatus incorporating a laser beam optical scanning device as an image printing device such as a laser printer, digital copier and the like.
2. Description of the Related Art
In recent years the laser beam optical scanning devices incorporated in laser printers and digital copiers as image printing devices have become capable of printing at high resolution to improve image quality. Therefore, the permissible depth of focus has become shallower as the laser beam spot diameter has become smaller. When optical elements and their holders are subjected to thermal expansion due to environmental fluctuation and particularly heat generated by the optical device during use, the laser beam spot diameter increases due to the shifting of the light convergence position in a longitudinal direction on the scanned surface so as to exceed tolerances necessary to obtain high image quality.
Various conventional measures have been proposed to resolve the aforesaid disadvantages. For example, U.S. Pat. No. 5,241,174 and U.S. Pat. No. 5,122,658 disclose solutions wherein a focusing lens is moved to an optimum position based on the convergence state of a laser beam detected by a sensor element provided with a single grating filter. U.S. Pat. No. 5,231,280 discloses a solution wherein a focusing lens is moved to an optimum position based on the convergence state of a laser beam detected by a sensor comprising a photoelectric conversion element and a knife edge disposed at a different position relative to the scanned surface.
In such conventional devices, however, focusing errors occur when a suitable amount of position displacement is not obtained due to the influence of noise and the like contaminating the detector. Japanese Laid-Open Patent Application No. HEI 2-296210 discloses a solution wherein a focusing lens is moved to a focus position based on design when the shifting of the convergence position cannot be adequately detected. The convergence state is not necessarily satisfactory even when the focusing lens is returned to a design-based focus position, however.